alpha level  The probability of incorrectly rejecting the null hypothesis that is used by a researcher to decide whether an outcome of a study is statistically significant (most commonly, researchers use a probability of .05).




alternative explanation  Part of causal inference; a potential alternative cause of an observed relationship between variables.




analysis of variance  See F test.




archival data  Information that is obtained from stored records including written, video, audio, and digital sources.




archival research  The use of existing sources of information for research. Sources include statistical records, survey archives, and written records.




attrition  The loss of subjects who decide to leave an experiment. See mortality.




autonomy (Belmont Report)  Principle that individuals in research investigations are capable of making a decision of whether to participate.




bar graph  A visual presentation that uses bars to depict frequencies of responses, percentages, or means in two or more groups.




baseline  In a single case design, the subject's behavior during a control period before introduction of the experimental manipulation.




beneficence (Belmont Report)  Principle that research should have beneficial effects while minimizing any harmful effects.




betweensubjects design  An experiment in which different subjects are assigned to each group. Also called independent groups design.




carryover effect  A problem that may occur in repeated measures designs if the effects of one treatment are still present when the next treatment is given.




case study  A descriptive account of the behavior, past history, and other relevant factors concerning a specific individual.




ceiling effect  Failure of a measure to detect a difference because it was too easy (also see floor effect).




central tendency  A single number or value that describes the typical or central score among a set of scores.




cluster sampling  A probability sampling method in which existing groups or geographic areas, called clusters, are identified. Clusters are randomly sampled and then everyone in the selected clusters participates in the study.




coding system  A set of rules used to categorize observations.




cohort  A group of people born at about the same time and exposed to the same societal events; cohort effects are confounded with age in a crosssectional study.




conceptual replication  A type of replication of research using different procedures for manipulating or measuring the variables.




conclusion validity  Extent to which the conclusions about the relationships among variables reached on the basis of the data are correct.




concurrent validity  The construct validity of a measure is assessed by examining whether groups of people differ on the measure in expected ways.




confederate  A person posing as a participant in an experiment who is actually part of the experiment.




confidence interval  An interval of values within which there is a given level of confidence (e.g., 95%) where the population value lies.




confounding  An uncontrolled variable varies systematically with an independent variable; it is impossible to separate the effect of the independent variable from the confounding variable.




confounding variable  A variable that is not controlled in a research investigation. In an experiment, the experimental groups differ on both the independent variable and the confounding variable.




construct validity  The degree to which a measurement device accurately measures the theoretical construct it is designed to measure.




content analysis  Systematic analysis of recorded communications.




content validity  An indicator of construct validity of a measure in which the content of the measure is compared to the universe of content that defines the construct.




control series design  An extension of the interrupted time series quasiexperimental design in which there is a comparison or control group.




convergent validity  The construct validity of a measure is assessed by examining the extent to which scores on the measure are related to scores on other measures of the same construct or similar constructs.




correlation coefficient  An index of how strongly two variables are related to each other.




correlational method  See nonexperimental method.




counterbalancing  A method of controlling for order effects in a repeated measures design by either including all orders of treatment presentation or randomly determining the order for each subject.




covariation of cause and effect  Part of causal inference; observing that a change in one variable is accompanied by a change in a second variable.




criterion variable  The variable/score that is predicted based upon an individual's score on another variable (the predictor variable). Conceptually similar to a dependent variable.




Cronbach's alpha  An indicator of internal consistency reliability assessed by examining the average correlation of each item (question) in a measure with every other question.




crosssectional method  A developmental research method in which persons of different ages are studied at only one point in time; conceptually similar to an independent groups design.




curvilinear relationship  A relationship in which changes in the values of the first variable are accompanied by both increases and decreases in the values of another variable.




debriefing  Explanation of the purposes of the research that is given to participants following their participation in the research.




deception  Misinformation that a participant receives during a research investigation.




degrees of freedom (df)  A concept used in tests of statistical significance; the number of observations that are free to vary to produce a known outcome.




demand characteristics  Cues that inform the subject how he or she is expected to behave.




dependent variable  The variable that is the subject's response to, and dependent on, the level of the manipulated independent variable.




descriptive statistics  Statistical measures that describe the results of a study; descriptive statistics include measures of central tendency (e.g., mean), variability (e.g., standard deviation), and correlation (e.g., Pearson r).




discriminant validity  The construct validity of a measure is assessed by examining the extent to which scores on the measure are not related to scores on conceptually unrelated measures.




doubleblind  A procedure wherein both the experimenter and the participant are unaware of whether the participant is in the experimental (treatment) or the control condition.




effect size  The extent to which two variables are associated. In experimental research, the magnitude of the impact of the independent variable on the dependent variable.




electroencephalogram (EEG)  A measure of the electrical activity of the brain.




electromyogram (EMG)  A measure of the electrical activity of muscles, including muscle tension.




empiricism  Use of objective observations to answer a question about the nature of behavior.




error variance  Random variability in a set of scores that is not the result of the independent variable. Statistically, the variability of each score from its group mean.




exact replication  A type of replication of research using the same procedures for manipulating and measuring the variables that were used in the original research.




experimental control  Eliminating the influence of an extraneous variable on the outcome of an experiment by keeping the variable constant in the experimental and control groups.




experimental method  A method of determining whether variables are related, in which the researcher manipulates the independent variable and controls all other variables either by randomization or by direct experimental control.




experimenter bias (expectancy effects)  Any intentional or unintentional influence that the experimenter exerts on subjects to confirm the hypothesis under investigation.




external validity  The degree to which the results of an experiment may be generalized.




extraneous variable  See third variable.




F test (analysis of variance)  A statistical significance test for determining whether two or more means are significantly different. F is the ratio of systematic variance to error variance.




face validity  The degree to which a measurement device appears to accurately measure a variable.




factorial design  A design in which all levels of each independent variable are combined with all levels of the other independent variables. A factorial design allows investigation of the separate main effects and interactions of two or more independent variables.




falsifiability  The principle that a good scientific idea or theory should be capable of being shown to be false when tested using scientific methods.




field experiment  An experiment that is conducted in a natural setting rather than in a laboratory setting.




filler items  Items included in a questionnaire measure to help disguise the true purpose of the measure.




floor effect  Failure of a measure to detect a difference because it was too difficult (also see ceiling effect).




frequency distribution  An arrangement of a set of scores from lowest to highest that indicates the number of times each score was obtained.




frequency polygon  A graphic display of a frequency distribution in which the frequency of each score is plotted on the vertical axis, with the plotted points connected by straight lines.




functional MRI  Magnetic resonance imaging uses a magnet to obtain scans of structures of the brain. Functional magnetic resonance imaging (fMRI) provides information on the amount of activity in different brain structures.




galvanic skin response (GSR)  The electrical conductance of the skin, which changes when sweating occurs.




haphazard (convenience) sampling  Selecting subjects in a haphazard manner, usually on the basis of availability, and not with regard to having a representative sample of the population; a type of nonprobability sampling.




histogram  Graphic representation of a frequency distribution using bars to represent each score or group of scores.




history effect  As a threat to the internal validity of an experiment, refers to any outside event that is not part of the manipulation that could be responsible for the results.




hypothesis  An assertion about what is true in a particular situation; often, a statement asserting that two or more variables are related to one another.




independent groups design  An experiment in which different subjects are assigned to each group. Also called betweensubjects design.




independent variable  The variable that is manipulated to observe its effect on the dependent variable.




inferential statistics  Statistics designed to determine whether results based on sample data are generalizable to a population.




informed consent  In research ethics, the principle that participants in an experiment be informed in advance of all aspects of the research that might influence their decision to participate.




Institutional Review Board (IRB)  An ethics review committee established to review research proposals. The IRB is composed of scientists, nonscientists, and legal experts.




instrument decay  As a threat to internal validity, the possibility that a change in the characteristics of the measurement instrument, including human observers, is responsible for the results.




interaction  Situation in which the effect of one independent variable on the dependent variable changes, depending on the level of another independent variable.




internal consistency reliability  Reliability assessed with data collected at one point in time with multiple measures of a psychological construct. A measure is reliable when the multiple measures provide similar results.




internal validity  The certainty with which results of an experiment can be attributed to the manipulation of the independent variable rather than to some other, confounding variable.




interrater reliability  An indicator of reliability that examines the agreement of observations made by two or more raters (judges).




interrupted time series design  A design in which the effectiveness of a treatment is determined by examining a series of measurements made over an extended time period both before and after the treatment is introduced. The treatment is not introduced at a random point in time.




interval scale  A scale of measurement in which the intervals between numbers on the scale are all equal in size.




interviewer bias  Intentional or unintentional influence exerted by an interviewer in such a way that the actual or interpreted behavior of respondents is consistent with the interviewer's expectations.




itemtotal correlation  The correlation between scores on individual items with the total score on all items of a measure.




IV × PV design  A factorial design that includes both an experimental independent variable (IV) and a nonexperimental participant variable (PV).




justice (Belmont Report)  Principle that all individuals and groups should have fair and equal access to the benefits of research participation as well as potential risks of research participation.




Latin square  A technique to control for order effects without having all possible orders.




longitudinal method  A developmental research method in which the same persons are observed repeatedly as they grow older; conceptually similar to a repeated measures design.




main effect  The direct effect of an independent variable on a dependent variable.




manipulation check  A measure used to determine whether the manipulation of the independent variable has had its intended effect on a subject.




matched pairs design  A method of assigning subjects to groups in which pairs of subjects are first matched on some characteristic and then individually assigned randomly to groups.




maturation effect  As a threat to internal validity, the possibility that any naturally occurring change within the individual is responsible for the results.




mean  A measure of central tendency, obtained by summing scores and then dividing the sum by the number of scores.




measurement error  The degree to which a measurement deviates from the true score value.




median  A measure of central tendency; the middle score in a distribution of scores that divides the distribution in half.




metaanalysis  A set of statistical procedures for combining the results of a number of studies in order to provide a general assessment of the relationship between variables.




minimal risk research  Research in which participants are exposed to risks that are no greater than those encountered in daily life or in routine physical or psychological tests.




mixed factorial design  A design that includes both independent groups (betweensubjects) and repeated measures (withinsubjects) variables.




mode  A measure of central tendency; the most frequent score in a distribution of scores.




moderator variable  A variable that influences the nature of the relationship between two other variables (an independent variable and a dependent variable). In a factorial design, the effect of the moderator variable is revealed as an interaction.




mortality  The loss of subjects who decide to leave an experiment. Mortality is a threat to internal validity when the mortality rate is related to the nature of the experimental manipulation.




multiple baseline design  Observing behavior before and after a manipulation under multiple circumstances (across different individuals, different behaviors, or different settings).




multiple correlation  A correlation between one variable and a combined set of predictor variables.




naturalistic observation  Descriptive method in which observations are made in a natural social setting. Also called field observation.




negative case analysis  In field observation, an examination of observations that do not fit with the explanatory structure devised by the researcher.




negative linear relationship  A relationship in which increases in the values of the first variable are accompanied by decreases in the values of the second variable.




no relationship  Outcome of research in which two variables are not related; changes in the first variable are not associated with changes in the second variable.




nominal scale  A scale of measurement with two or more categories that have no numerical (less than, greater than) properties.




nonequivalent control group design  A quasiexperimental design in which nonequivalent groups of subjects participate in the different experimental groups, and there is no pretest.




nonequivalent control group pretestposttest design  A quasiexperimental design in which nonequivalent groups are used, but a pretest allows assessment of equivalency and pretestposttest changes.




nonexperimental method  Use of measurement of variables to determine whether variables are related to one another. Also called correlational method.




nonprobability sampling  Type of sampling procedure in which one cannot specify the probability that any member of the population will be included in the sample.




null hypothesis  The hypothesis, used for statistical purposes, that the variables under investigation are not related in the population, that any observed effect based on sample results is due to random error.




onegroup posttestonly design  A quasiexperimental design that has no control group and no pretest comparison; a very poor design in terms of internal validity.




onegroup pretestposttest design  A quasiexperimental design in which the effect of an independent variable is inferred from the pretestposttest difference in a single group.




operational definition  Definition of a concept that specifies the method used to measure or manipulate the concept.




order effect  In a repeated measures design, the effect that the order of introducing treatment has on the dependent variable.




ordinal scale  A scale of measurement in which the measurement categories form a rank order along a continuum.




panel study  Research in which the same sample of subjects is studied at two or more points in time, usually to assess changes that occur over time.




partial correlation  The correlation between two variables with the influence of a third variable statistically controlled for.




participant observation  A technique of observing a situation wherein the observer takes an active role in the situation.




participant (subject) variable  A characteristic of the research participant such as gender, age, personality, or ability.




Pearson productmoment correlation coefficient  A type of correlation coefficient used with interval and ratio scale data. In addition to providing information on the strength of relationship between two variables, it indicates the direction (positive or negative) of the relationship.




peer review  The process of judging the scientific merit of research through review by other scientists with the expertise to evaluate the research.




pie chart  Graphic display of data in which frequencies or percentages are represented as "slices" of a pie.




pilot study  A smallscale study conducted prior to conducting an actual experiment; designed to test and refine procedures.




placebo group  In drug research, a group given an inert substance to assess the psychological effect of receiving a treatment.




plagiarism  Presentation of another person's work as your own instead of properly citing the actual source.




population  The defined group of individuals from which a sample is drawn.




positive linear relationship  A relationship in which increases in the values of the first variable are accompanied by increases in the values of the second variable.




posttestonly design  A true experimental design in which the dependent variable (posttest) is measured only once, after manipulation of the independent variable.




power  The probability of correctly rejecting the null hypothesis.




practice effect  Improvement in participant performance with repeated testing.




prediction  An assertion concerning what will occur in a particular research investigation.




predictive validity  The construct validity of a measure is assessed by examining the ability of the measure to predict a future behavior.




predictor variable  A variable that is used to make a prediction of an individual's score on another variable (the criterion variable). Conceptually similar to an independent variable.




pretestposttest design  A true experimental design in which the dependent variable is measured both before (pretest) and after (posttest) manipulation of the independent variable.




probability  The likelihood that a given event (among a specific set of events) will occur.




probability sampling  Type of sampling procedure in which one is able to specify the probability that any member of the population will be included in the sample.




program evaluation  Research designed to assess procedures (e.g., social reforms, innovations) that are designed to produce certain changes or outcomes in a target population.




propensity score matching  A method of pairing individuals for assignment to a treatment and control condition based upon a combination of scores on participant variables.




pseudoscience  Claims that are made on the basis of evidence that, despite appearances, is not based on the principles of the scientific method.




psychobiography  A type of case study in which the life of an individual is analyzed using psychological theory.




purposive sample  A type of haphazard sample conducted to obtain predetermined types of individuals for the sample.




quasiexperimental design  A type of design that approximates the control features of true experiments to infer that a given treatment did have its intended effect.




quota sampling  A sampling procedure in which the sample is chosen to reflect the numerical composition of various subgroups in the population. A haphazard sampling technique is used to obtain the sample.




random assignment  Use of a random "chance" procedure (such as a random number generator or coin toss) to determine the condition in which an individual will participate.




randomization  Controlling for the effects of extraneous variables by ensuring that the variables operate in a manner determined entirely by chance.




ratio scale  A scale of measurement in which there is an absolute zero point, indicating an absence of the variable being measured. An implication is that ratios of numbers on the scale can be formed (generally, these are physical measures such as weight or timed measures such as duration or reaction time).




reactivity  A problem of measurement in which the measure changes the behavior being observed.




regression equation  A mathematical equation that allows prediction of one behavior when the score on another variable is known.




regression toward the mean  Also called statistical regression; principle that extreme scores on a variable tend to be closer to the mean when a second measurement is made.




reliability  The degree to which a measure is consistent.




repeated measures design  An experiment in which the same subjects are assigned to each group. Also called withinsubjects design.




replication  Repeating a research study to determine whether the results can be duplicated.




research hypothesis  The hypothesis that the variables under investigation are related in the population—that the observed effect based on sample data is true in the population.




response rate  The percentage of people selected for a sample who actually completed a survey.




response set  A pattern of response to questions on a selfreport measure that is not related to the content of the questions.




restriction of range  A problem when scores on a variable are limited to a small subset of their possible values; this makes it more difficult to identify relationships of the variable to other variables of interest.




reversal design  A single case design in which the treatment is introduced after a baseline period and then withdrawn during a second baseline period. It may be extended by adding a second introduction of the treatment. Sometimes called a "withdrawal" design.




riskbenefit analysis  Evaluation of the ethical implications of a research procedure by considering the benefits of the research in relation to the potential risks to participants.




roleplaying  An alternative to deception in which individuals indicate how they or others would respond to an independent variable.




sampling  The process of choosing members of a population to be included in a sample.




sampling distribution  Theoretical distribution of the frequency of all possible outcomes of a study conducted with a given sample size.




sampling frame  The individuals or clusters of individuals in a population who might actually be selected for inclusion in the sample.




scatterplot  Graphic representation of each individual's scores on two variables. The score on the first variable is found on the horizontal axis and score on the second variable is found on the vertical axis.




selection differences  Differences in the type of subjects who make up each group in an experimental design; this situation occurs when participants elect which group they are to be assigned to.




sensitivity  The ability of a measure to detect differences between groups.




sequential method  A combination of the crosssectional and longitudinal design to study developmental research questions.




significance level  See alpha level.




simple main effect  In a factorial design, the effect of one independent variable at a particular level of another independent variable.




simple random sampling  A sampling procedure in which each member of the population has an equal probability of being included in the sample.




single case experiment  An experiment in which the effect of the independent variable is assessed using data from a single participant.




Solomon fourgroup design  Experimental design in which the experimental and control groups are studied with and without a pretest.




splithalf reliability  A reliability coefficient determined by the correlation between scores on half of the items on a measure with scores on the other half of a measure.




standard deviation  The average deviation of scores from the mean (the square root of the variance).




statistical significance  Rejection of the null hypothesis when an outcome has a low probability of occurrence (usually .05 or less) if, in fact, the null hypothesis is correct.




stratified random sampling  A probability sampling method in which a population is divided into subpopulation groups called strata; individuals are then randomly sampled from each of the strata.




structural equation modeling  Statistical techniques that are used to evaluate a proposed set of relationships among variables.




systematic observation  Observations of one or more specific variables, usually made in a precisely defined setting.




systematic variance  Variability in a set of scores that is the result of the independent variable; statistically, the variability of each group mean from the grand mean of all subjects.




temporal precedence  Part of causal inference; the cause occurs before the effect.




testing effect  A threat to internal validity in which taking a pretest changes behavior without any effect on the independent variable.




testretest reliability  A reliability coefficient determined by the correlation between scores on a measure given at one time with scores on the same measure given at a later time.




theory  A systematic, coherent, and logical set of ideas about a particular topic or phenomenon that serves to organize and explain data and generate new knowledge.




third variable  In descriptions of the relationship between two variables, a third variable is any other variable that is extraneous to the two variables of interest. True experiments control for the possible influence of third variables.




true score  An individual's actual score on a variable being measured, as opposed to the score the individual obtained on the measure itself.




ttest  A statistical significance test used to compare differences between means.




Type I error  An incorrect decision to reject the null hypothesis when it is true.




Type II error  An incorrect decision to accept the null hypothesis when it is false.




validity  See construct validity, external validity, internal validity.




variability  The amount of dispersion of scores about some central value.




variable  Any event, situation, behavior, or individual characteristic that varies—that is, has at least two values.




variance  A measure of the variability of scores about a mean; the mean of the sum of squared deviations of scores from the group mean.




withinsubjects design  An experiment in which the same subjects are assigned to each group. Also called repeated measures design.
